(a) Field of the Invention
The present invention relates to a method for circumferential welding of a line pipe for transporting CO.sub.2 -containing petroleum or natural gas or transporting CO.sub.2 and, more particularly, it relates to a circumferential welding method excellent in preferential corrosion resistance, low-temperature toughness and crack resistance of weld metal and also relates to weld wire for suitably carrying out the welding method.
(b) Description of the Prior Art
There are published literatures concerning preferential corrosion on a weld portion of low alloy steel, as follows.
(1) Statement that the difference between the Ni content of base material and the Ni content of weld metal gives an influence on the local corrosion of a weld joint portion in a frozen sea region (by Takashi Abe et al.; "Iron and Steel", Vol. 72, No. 12, s1266, 1986). PA0 (2) Statement that Ni and Cu represented by PA0 (3) Statement that use of a low alloy weld rod containing Cu and Ni is effective on the prevention of the preferential corrosion of a circumferential weld portion of a carbon steel pipe (by Hideaki Ko; "Material" Vol. 38, No. 424, p62-p68, 1989). PA0 (4) Statement that addition of Ni and Mo is effective on the prevention of the preferential corrosion of a vertical seam weld portion of a welded pipe (by Suga et al.; Japanese Patent Laid-open No. 3-170641). PA0 C; 0.01 to 0.15% PA0 Si; 0.20 to 1.00% PA0 Mn; 0.40 to 2.00% PA0 Cu; .ltoreq.2.50% PA0 Ni; 0.50 to 2.50% PA0 .DELTA.Cu+Ni; .gtoreq.0.50% PA0 P.sub.CM ; .ltoreq.0.25% PA0 Mo; .ltoreq.1.05% or .DELTA.Mo; .gtoreq.0.03% PA0 Ti; .ltoreq.0.25% PA0 1 The chemical components of the pipe base material contain (% by weight): PA0 C; 0.03 to 0.15% PA0 Si; 0.05 to 0.50% PA0 Mn; 0.50 to 2.00% PA0 Al; 0.005 to 0.10% PA0 Cu; 0.05 to 2.0% PA0 Ni; 0.05 to 2.0% PA0 Cr; 0.05 to 2.0% PA0 Mo; 0.05 to 1.0% PA0 Nb; 0.005 to 0.20% PA0 V; 0.005 to 0.20% PA0 Ti; 0.005 to 0.20% PA0 B; 0.0005 to 0.0020% PA0 Ca; 0.0005 to 0.0050; PA0 2 the chemical components of the weld wire contain (% by weight); PA0 C; 0.01 to 0.15% PA0 Si; 0.20 to 1.20% PA0 Mn; 0.60 to 2.50% PA0 Cu; .ltoreq.3.00% PA0 Ni; 0.50 to 3.00% PA0 Mo; .ltoreq.1.10% PA0 Ti; .ltoreq.0.30% PA0 3 The welding conditions are: PA0 shield gas; 100% CO.sub.2 or Ar+(from 5 to 40% of) CO.sub.2 PA0 wire diameter; 0.8 to 1.6 mm PA0 welding current; 100 to 500 A PA0 arc voltage; 15 to 45 V PA0 welding speed; 5 to 150 cm/min PA0 welding attitude; all position. PA0 C; 0.01 to 0.15% PA0 Si; 0.20 to 1.20% PA0 Mn; 0.60 to 2.50% PA0 Cu; .ltoreq.3.00% PA0 Ni; 0.50 to 3.00% PA0 C; 0.01 to 0.15% PA0 Si; 0.20 to 1.00% PA0 Mn; 0.40 to 2.00% PA0 Cu; .ltoreq.2.50% PA0 Ni; 0.50 to 2.50% PA0 P.sub.CM; .ltoreq. 0.25% PA0 Mo; .ltoreq.1.05% PA0 .DELTA.Mo; .ltoreq.0.03% PA0 Ti; .ltoreq.0.25% PA0 Inevitable impurities; When the amounts of inevitable impurities are respectively in the aforementioned ranges, there is no deterioration of preferential corrosion characteristic and mechanical properties of the weld metal. If the amounts of inevitable impurities are respectively larger than the aforementioned ranges, defects such as lowering of welding workability (Al, Zr, Ca), occurrence of a failure in welding (P, S, B, N), deterioration of mechanical properties (Al, Cr, Nb, V, O, N) and the like arise. PA0 C; 0.03 to 0.15% PA0 Si; 0.05 to 0.50% PA0 Mn; 0.50 to 2.00% PA0 Al; 0.005 to 0.10% PA0 Cu; 0.05 to 2.0% PA0 Ni; 0.05 to 2.0% PA0 Cr; 0.05 to 2.0% PA0 Mo; 0.05 to 1.0% PA0 Nb; 0.005 to 0.20% PA0 V; 0.005 to 0.20% PA0 Ti; 0.005 to 0.20% PA0 B; 0.0005 to 0.0020% PA0 Ca; 0.0005 to 0.0050% PA0 C; 0.01 to 0.15% PA0 Si; 0.20 to 1.20% PA0 Mn; 0.60 to 2.50% PA0 Cu; .ltoreq.3.00% PA0 Ni; 0.50 to 3.00%. PA0 Mo; .ltoreq.1.10% PA0 Ti; .ltoreq.0.30% PA0 Inevitable impurities; It is preferable that the amounts of inevitable impurities are respectively as small as possible. Particularly, when inevitable impurities satisfy the ranges: P.ltoreq.0.030%; S.ltoreq.0.030%; Al.ltoreq.0.05%; N.ltoreq.0.01%; Nb.ltoreq.0.02%; V.ltoreq.0.02%; Cr.ltoreq.0.05%; Zr.ltoreq.0.05%; O.ltoreq.0.02%; and B.ltoreq.0.002%, there is no deterioration of preferential corrosion resistance and mechanical properties of the weld metal. If the amounts of inevitable impurities are respectively larger than the aforementioned ranges, defects such as lowering of welding workability (Al, Zr), occurrence of a failure in welding (P, S, B, N), deterioration of mechanical properties (Al, Cr, Nb, V, O, N) and the like arise. PA0 Shield gas: The composition of shield gas availably used for CO.sub.2 welding or MAG (metal active gas) welding is employed. PA0 Wire diameter: The diameter of a welding wire availably used correspondingly to the open-top form and the welding attitude is employed. PA0 Welding current, arc voltage, welding speed: The welding current, the arc voltage and the welding speed generally used in accordance with the open-top form, the welding attitude and the wire diameter are employed. PA0 Welding attitude: All position
3.8.DELTA.Cu+1.1.DELTA.Ni+0.3 PA1 (.DELTA.; content in weld metal--content in base material) PA1 (.DELTA.: content in weld metal--content in base material)
give an influence on preferential corrosion characteristic in the case of the local corrosion of a weld portion of steel in a frozen sea region (by Kitaro Ito et al.; "Iron and Steel", Vol. 72, No. 12, s1265, 1986).
As described above, there have been found a method of adding Ni and Cu to improve the preferential corrosion of weld metal in a corrosion environment of seawater containing oxygen such as a frozen sea and a method of adding Ni and Mo to improve the preferential corrosion characteristic of a vertical seam portion of a welded steel pipes. There is not yet any knowledge that addition of Ni and Mo is effective for the prevention of the preferential corrosion of a circumferential weld portion of a line pipe used in a corrosion environment containing CO.sub.2. There is not yet any gas-shield arc welding method based on the consideration of the hardness of the weld metal, the crack resistance thereof and the like. Further, there is not yet any method for solving the problem specifically in the relation between the weld metal and the base material.
When a welded steel pipe or a seamless steel pipe is used for transporting CO.sub.2 -containing petroleum or natural gas or transporting CO.sub.2, so-called preferential corrosion of a weld portion in which circumferential weld metal is corroded preferentially may occur. This is a phenomenon that the difference in chemical components and structure between the weld metal and the base material makes the weld metal portion electrochemically poor to thereby preferentially corrode the weld metal portion.
In the case of a line pipe used in the aforementioned environment, any circumferential welding method based on the consideration of the preferential corrosion has been not discussed. In a real environment, this kind preferential corrosion is often put in a problem awaiting solution so that the discussion thereof is expected today.